Merge branch 'ab/test-env-helper'

Remove "git env--helper" and demote it to a test-tool subcommand.

* ab/test-env-helper:
  env-helper: move this built-in to "test-tool env-helper"

diff --cc .gitignore
Simple merge

diff --cc Makefile
Simple merge

diff --cc git.c
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge